Life as a Physicist

The Way You Look at the World Will Change… Soon December 2, 2011

We are coming up on one of those “lucky to be alive to see this” moments. Sometime in the next year we will all know, one way or the other, if the Standard ModelHiggs exists. Or it does not exist. How we think fundamental physics will change. I can’t understate the importance of this. And the first strike along this path will occur on December 13th.

If it does not exist that will force us to tear down and rebuild – in some totally unknown way – our model of physics. Our model that we’ve had for 40+ years now. Imagine that – 40 years and now that it finally meets data… poof! Gone. Or, we will find the Higgs, and we’ll have a mass. Knowing the mass will be in itself interesting, and finding the Higgs won’t change the fact that we still need something more than the Standard Model to complete our description of the universe. But now every single beyond-the-standard model theory will have to incorporate not only electrons, muons, quarks, W’s, Z’s, photons, gluons – at their measured masses, but a Higgs too with the appropriate masses we measure!

Ok, this takes a second to explain. First, when we look for the Higgs we do it as a function of its mass – the theory does not predict exactly how massive it will be. Second, the y-axis is the rate at which the Higgs is produced. When we look for it at a certain mass we make a statement “if the Higgs exists at mass 200 GeV/c2, then it must be happening at a rate less than 0.6 or we would have seen it.” I read the 0.6 off the plot by looking at the placement of the solid black line with the square points – the observed upper limit. The rate, the y-axis, is in funny units. Basically, the red line is the rate you’d expect if it was a standard model Higgs. The solid black line with the square points on it is the combined LHC exclusion line. Combined means ATLAS + CMS results. So, anywhere the solid black line dips below the red horizontal line means that we are fairly confident that the Standard Model Higgs doesn’t exist (BTW – even fairly confident has a very specific meaning here: we are 95% confident). The hatched areas are the areas where the Higgs has already been ruled out. Note the hatched areas at low mass (100 GeV or so) – those are from other experiments like LEP.

Now that is done. A fair question is where would we expect to find the Higgs. As it turns out, a Standard Model Higgs will mostly likely occur at low masses – exactly that region between 114 GeV/c2 and 140 GeV/c2. There isn’t a lot of room left for the Higgs to hide there!! These plots are with 2 fb-1 of data. Both experiments now have about 5 fb-1 of data recorded. And everyone wants to know exactly what they see. Heck, while in each experiment we basically know what we see, we desperately want to know what the other experiment sees. The first unveiling will occur at a joint seminar at 2pm on December 13th. I really hope it will be streamed on the web, as I’ll be up in Whistler for my winder ski vacation!

So what should you look for during that seminar (or in the talks that will be uploaded when the seminar is given)? The above plot will be a quick summary of what the status of the experiments. Each experiment will have an individual one. The key thing to look for is where the dashed line and the solid line deviate significantly. The solid line I’ve already explained – that says that for the HIggs of a particular mass if it is there, it must be at a rate less than what is shown. Now, the dashed line is what we expect – given everything was right – and the Higgs didn’t exist at that mass – that is how good we expect to be. So, for example, right around the 280 GeV/C2 level we expect to be able to see a rate of about 0.6, and that is almost exactly what we measure. Now look down around 120-130 GeV/c2. There you’ll notice that the observed line is well above the solid line. How much – well, it is just along the edge of the yellow band – which means 2 sigma. 2 sigma isn’t very much – so this plot has nothing to get very interested yet. But if one of the plots shown over the next year has a more significant excursion, and you see it in both experiments… then you have my permission to get a little excited. The real test will be if we can get to a 5 sigma excursion.

This seminar is the first step in this final chapter of the old realm of particle physics. We are about to start a new chapter. I, for one, can’t wait!

N.B. I’m totally glossing over the fact that if we do find something in the next year that looks like a Higgs, it will take us sometime to make sure it is a Standard Model Higgs, rather than some other type of Higgs! 2nd order effect, as they say. Also, in that last long paragraph, the sigma’s I’m talking about on the plot and the 5 sigma discovery aren’t the same – so I glossed over some real details there too (and this latter one is a detail I sometimes forget, much to my embarrassment at a meeting the other day!).

Yes – that is correct. The reason is that those two numbers are answering two different questions. If the first case (the limit plot you are looking at) you are asking if you rule out the Standard Model, basically – so you are including uncertaintys on the Higgs boson cross section (given its mass) – that come from theory, say. In the latter case, just the number, you are saying “well, I have no idea what the higgs boson cross section is, I just know that it is at mass 140 GeV – so what are the chances the background could have fluctuated to emulate that 140 GeV?” – which is different. Now, in reality, the two should be very very close to each other. And if they aren’t then you need to ask some questions. 🙂 So, this is a minor point and shouldn’t distract from the actual message, though it is a real point.

Watching this week’s Higgs hoopla has been fun. One thing I find missing from the presentations is some sort of internal calibration of the exclusion plots. How can I check, by eye, if the analysis is done correctly?

Put another way – I think I know how to eyeball data taken with minimal bias; I’m looking for a resonance that stands up and away from nearby background with obvious statistical significance.

Is there anything similar that will let the lay-physicist tell that the background subtraction is roughly correct?

Hi Charlie,
Yes and no. It really depends on what channel you are searching in and it depends on how the data is presented. For example, the recent CDF dijet bump. In background subtracted form, the bump looks very convincing. But if you don’t background subtract it, you see it is on a sharply falling (huge) distribution. So what you want can fool you!

In the Higgs case, a dijet mass bump will be very spread out, but a mass bump made up of di-muons or di-photons will show up very cleanly – this is strictly a function of the resolution of the objects that you are looking at (ok, the underlying backgrounds too, but that isn’t so relavent to this discussion).

Many analyses now use multi-variate techniques – neural networks or decision trees. The point behind this is that there is more information in each event other than the di-jet or di-photon peak. Perhaps the boost of reconstructed Higgs, for example, is dramatically different when you have a real Higgs vs when you just happen to have two random photons in the event. In your race to be first, you’d be crazy to ignore that extra selection handle. There will be other handles.

The problem is the output of an NN or a DT doesn’t look like a nice mass bump. Very hard to trust by eye – unless you’ve been working on the analysis – the NN or DT or whatever acts a bit like a black box. And these techniques are used at several stages in the analysis (i.e. how do you select a b-tag, how do you select an electron/photon, etc.).

The last thing to remember is – this isn’t a discovery – our statistics are too small for that so far. So if you could *clearly* see it with your eye, then our claim of only 3 sigma significance would either be bull or we had somehow arranged our plots to make the data look better than it really is. 🙂

I dont work in physics anymore, for years. I am glad to know that the
125 GeV fits the formula of 10^n GeV +/- 2^i 5^j MeV as the particles of the SM do. Some symmetry. (The LHC people should check those values also as far as I could tell, for anything wierd in the data.)